1. Field of the Invention
The present invention relates to an optical circuit for use in wavelength-division multiplexed optical communication in which a plurality of signal beams having a plurality of wavelengths are propagated on a single transmission line, and particularly to an optical fiber amplifier for batch amplification of multiple wavelengths.
2. Description of the Related Art
In recent years, WDM (Wavelength Division Multiplexer) transmission technology has been receiving attention as a method for high-speed, large-capacity communication in which signals of a plurality of wavelengths are transmitted in a batch.
WDM transmission enables the transmission of a large volume of data at a low transmission speed by transmitting on a plurality of wavelengths. For example, a system in which four wavelengths are sent as a batch at a transmission speed of 2.4 Gbps achieves substantially the same transmission volume as a system in which one wavelength is sent at a transmission speed of 10 Gbps. At the current level of the art, multiplexing wavelengths is technologically more amenable to commercialization than improving transmission speed, and as a result, WDM transmission technology is undergoing considerable development as a means of increasing transmission capacity.
In WDM transmission, the signal level on each channel must be made uniform in order to maintain equal transmission performance on each channel. Signal level is determined by such factors as the wavelength dependency of the transmission line insertion loss or the wavelength dependency of the gain of a optical fiber amplifier, and differences between gain on each wavelength (hereinbelow referred to as "gain flatness") must therefore be reduced to a low level. The gain flatness of an optical fiber amplifier is influenced by fluctuation in gain. Normally, the output level of an optical fiber amplifier is held uniform by ALC (Automatic Level Control). Since output level is fixed despite fluctuation in input level, gain undergoes change, and gain flatness is affected. Experimentation shows that gain flatness deteriorates approximately 0.3 dB in a case in which gain varies 1 dB in a wavelength band of about 1545-1560 nm.
Assuming that the output level for each channel in an optical transmitter is fixed, causes of fluctuation in the input level to an optical fiber amplifier can be broadly divided between fluctuation in propagation loss on a transmission line and change in the number of transmission channels.
Fluctuation in output level resulting from changes in the number of transmission channels has a particularly large effect on gain flatness. The output of an optical fiber amplifier is commonly controlled as the total optical output power, and when the input signal power changes in a case of fixed output control, the gain of the optical fiber amplifier changes. For example, in the case of 16-channel WDM transmission in which the transmission power of each channel is equal, a change in the number of waves of the transmission channel from 16 channels to one channel causes the input level to drop 12 dB (1/16). Because the output power is fixed, the gain of the optical fiber rises 12 dB, and as a result, gain flatness deteriorates by approximately 3.6 dB.
When considering a multiple-stage relay, the minimum possible gain flatness (1 dB or less) is preferable for maintaining transmission performance, and the technique for controlling gain with respect to changes in the number of channels is therefore crucial. In the prior art, output level was determined by inputting channel number information to a control circuit of the optical fiber amplifier by electrical signals.
For example, if the total input power is -8 dBm and the total output power is +20 dBm (gain 28 dB) for 16 channels, control must be effected such that total input power is -11 dBm and total output power is +17 dBm (gain 28 dB) during transmission of 8 channels.
FIG. 1 and FIG. 2 are block diagrams showing the constructions of optical fiber amplifiers of the prior art.
In the prior-art example shown in FIG. 1, channel number information signal S201 is inputted to control circuit 204 by a system distinct from the optical fiber transmission line, and control circuit 204, in accordance with the number of channels indicated by channel number information signal S201, determines the amplitude of optical signal amplifier 201 that amplifies optical signals transmitted on the optical fiber transmission line.
The prior-art example shown in FIG. 2 raises the amplification capacity by using two optical signal amplifiers 211 and 212. In this case, channel number information signal S201 is inputted to control circuit 204 from a system distinct from optical fiber transmission line, and control circuit 204, in accordance with the number of channels indicated by channel number information signal S201, determines the amplitudes of optical signal amplifiers 211 and 212 that amplify optical signals transmitted on the optical fiber transmission line.
Constructions for inputting channel number information signal S201 to control circuit 204 from a system distinct from the optical fiber transmission line include a system by which the signal is inputted to control circuit 204 as an electrical signal from a line distinct from the optical fiber transmission line, and a system by which the signal is inputted on the optical fiber transmission line using a wavelength distinct from the signal wavelength (for example, with the signal in the 1.55-.mu.m band and the channel information in the 1.31-.mu.m band), OE-converted, and then inputted to control circuit 204.
Recent years have seen great activity in the development of circuits (hereinbelow referred to as an "ADD/DROP circuits") that are provided with an optical branching/coupling capability whereby optical signals are branched off midway on the transmission line and outputted to a separate branch line (DROP) or optical signals are added (ADD) to the transmission line from a separate branch line. In a case in which an ADD/DROP circuit is incorporated into the optical fiber amplifier, one possible construction for suppressing deterioration in NF (Noise Figure) due to the insertion loss of the ADD/DROP circuit involves dividing the optical fiber amplifier between a preceding stage and a succeeding stage and inserting the ADD/DROP circuit between them Because the number of channels changes between the preceding stage and succeeding stage in such a construction, information on channel number for each must be conferred to the circuits that control the preceding stage and succeeding stage.
FIG. 3 and FIG. 4 are block diagrams showing the construction of examples of prior-art optical fiber amplifiers that use the above-described ADD/DROP circuit. In these two cases, ADD/DROP circuit 209 is inserted between two optical signal amplifiers 211 and 212.
In the example of the prior art shown in FIG. 3, control circuit 241 causes optical signal amplification to be carried out at optical signal amplifier 211 in accordance with the channel number indicated by channel number information signal S201, and further, transfers the channel number indicated by channel number information signal S201. In addition to carrying out optical amplification, optical signal amplifier 211 employs optical signals of a wavelength distinct from the signal wavelength to output the channel number indicated by channel number information signal S201 to ADD/DROP circuit 209 by way of the optical transmission line. ADD/DROP circuit 209 transmits to optical signal amplifier 212 by way of the optical transmission line by carrying out optical branching/coupling and in addition outputs the channel number indicated by channel number information signal S201 to control circuit 242 as channel number information signal S202. Control circuit 242 directs optical signal amplification at optical signal amplifier 212 in accordance with the channel number indicated by channel number information signal S202.
The operation in the prior-art example shown in FIG. 4 is for the most part equivalent to that of the prior-art example shown in FIG. 3, but this example adopts a construction by which channel number information signal S201 is directly inputted to both control circuit 241 and to control circuit 242, the thus achieved optical amplification control being performed at optical signal amplifiers 211 and 212.